Dept. of Materials Science, Cornell University, Ithaca, NY
Nanoscale systems can display interesting and unique transformation kinetics that increase the structural complexity of the original material. Some of the most interesting nanoparticle morphologies and heterostructures in recent years have come from chemical transformations applied to nanoparticles, for example Kirkendall hollowing and partial cation exchange. Characterization of the transformation routes to form complex final structures is one of the major challenges in nanoscience. A more complete understanding of the transformation pathways would provide directions to improve synthesis techniques, leading to optimization of nanoparticles for use in applications. It also would provide insight into the control of nanoparticle chemical and physical properties. X-ray absorption spectroscopy (XAS) is a useful and innovative technique to study nanoscale systems where other characterization techniques fail due to resolution and sensitivity limits. Techniques such as x-ray diffraction (XRD), for example, are insufficient to analyze some nanoparticle systems that lack long-range order, particularly in intermediate phases. In XAS the XANES region provides details about sample geometric and electronic structure, and the EXAFS region provides short-range order, on the subnanometer scale, making it particularly important for nanoscale and amorphous materials. Through the combination of these techniques, along with TEM and DFT calculations, a thorough characterization and analysis of chemical transformations in nanoparticles is provided in this talk.
In this talk I will discuss our recent work studying chemical transformations of nanoparticles through x-ray absorption spectroscopy (XAS). I will discuss our work on the structural evolution and the diffusion processes which occur during the phase transformation of nanoparticle ε-Co to Co2P to CoP, from a reaction with tri-n-octylphosphine (TOP). Extended X-ray absorption fine structure (EXAFS) investigations were used to elucidate the changes in the local structure of cobalt atoms which occur as the chemical transformation progresses. Results from EXAFS, transmission electron microscopy, X-ray diffraction, and density functional theory calculations reveal that the inward diffusion of phosphorus is more favorable at the beginning of the transformation from ε-Co to Co2P by forming an amorphous Co-P shell, while retaining a crystalline cobalt core. When the major phase of the sample turns to Co2P, the diffusion processes reverse and cobalt atom out-diffusion is favored, leaving a hollow void, characteristic of the nanoscale Kirkendall effect.
I will also discuss our work using XAS to examine nickel and nickel phosphide nanoparticles, examining differences between the phases. EXAFS reveals that there is a significant amount of phosphorus in nickel samples, which appear as fcc nickel in XRD. This suggests that Ni-P intermediate phases retain the long range order of a phosphorus-poor structure despite excess Ni-P bonds with short-range ordering. We compare the long-range structural characterization by XRD to the short-range order displayed by EXAFS in order to investigate the limitations of XRD in nanoparticle characterization. This enables us to resolve the nanoparticle phase transition properties and diffusion mechanisms, which can lead to optimization of nanoparticle synthesis as well as nanoparticle use in device technology.